Many fabrication processes involve addition of layers or material to the surface of a specimen or subtraction of layers or material from the specimen to achieve desired optical, mechanical, electrical, electro-optical or any other physical feature that influences performance. Such processes are commonly known as surface modification processes. The surface on which the process occurs is commonly referred as a substrate. Surface modification processes have a wide range of applications including but not limited to material processing, semiconductor fabrication, energy generation and conversion devices, optical thin films, X-ray and UV optics, nanotechnology, meta-materials, general and 3D printing, etc. Surface properties such as roughness, relief, chemical homogeneity, uniformity and optical, electrical, thermal and mechanical properties can influence the properties or performance of the final device.
The process of thin film deposition is one example of an additive surface modification process. Thin films are thin material layers ranging from fractions of a nanometer to tens of microns in thickness. The substrate on which the thin films are formed (e.g., by deposition) can be a bare substrate, or may already have existing features. Various forms of physical vapor deposition (PVD) and chemical vapor deposition (CVD) are examples of thin film deposition processes. While most thin film deposition processes take place in vacuum, some processes may take place at atmospheric, or even higher, pressure conditions. The process usually consists of creating vapors of material by chemical of physical means such as plasma generation, evaporation, sputtering, sublimation or differential pressure and subsequent condensation of the vapors on the surface of the deposited substrate. Many other varieties of thin film deposition processes exist, such as implantation, laser-pulsed evaporation, atomic layer deposition, molecular epitaxy, spraying, thermal diffusion, surface oxidation, etc.
Thin film formation is a complex process requiring thorough control of the process parameters and, in some cases, control of film characteristics, such as optical, electrical, chemical composition, lattice orientation, thermal properties and mechanical stresses, while maintaining geometrically, stoichiometrically and structurally uniform films during film growth. Some of the materials that have been used to form thin films include amorphous and crystalline silicon, germanium, metal oxides, nitrides, carbides, other compound materials, a variety of semiconductors, dielectrics, metals, polymers, inks, toners and others. Thin-films are often deposited in multiple layers to achieve desired characteristics. In some cases, there is no definitive interface between separate layers as their properties gradually change from one layer to another. In other cases, the thickness and the properties of the layers are modulated, or vary in certain ways in the depth of the coating. Furthermore, the thickness and the properties of the layers can also be modulated or vary in all 2 or 3 dimensions as is the case in a variety of patterned coatings, general and 3D printing, MEMS, thin film microlenses, photonic crystals, waveguides, optical displays and many other optical products. Geometrical contact and shadow masks can be used to control the vapor distribution over the deposited substrate. A large variety of substrates can be used, including flexible substrates and substrates that can later be removed or etched away leaving the thin film coating to be self-supported, or to be transferred to another substrate.
Some other surface modification processes are caused by spontaneous means or by the environment at which the surface is exposed, such as ageing, corrosion, residue deposition, material fatigue, etc., and might need to be avoided or controlled in order to minimize their occurrence. Yet, some surface modification processes take place at the boundary between different solid and/or liquid materials, such as electro-chemical plating, Langmuir-Blodgett film formation, printing, plating, etc.
Subtractive surface modification processes are characterized by the intentional or unintentional removal of material from the substrate. As with the additive processes, there is a large variety of subtractive processes. One example of a subtractive surface modification process is surface etching. Etching is used in micro-fabrication to remove layers from the surface of a substrate (e.g., a wafer or another specimen).
Etching, stripping, and laser ablation are precise processes which require strict control of the process parameters in order to achieve the desired etching rate, etching profile and selectivity. In the ion etching process, control of the ratio of ion/reactive components in the plasma offers a convenient means to control the etching rate, the etching profile and selectivity. Another convenient means to control the process is achieved by applying bias voltages with different magnitude, profiles, waveforms, etc. Geometrical contact and shadow masks are often used to control the profile of layer subtraction from the substrate.
Another example of a subtractive surface modification process is the layer removal by mechanical scribing, such as grinding, polishing or spontaneous surface wear.
Surface modification processes typically change over time in an unwanted manner (i.e., randomly or systematically vary or fluctuate), causing the modified surface or specimen to either gradually deviate from the target values or otherwise unpredictably change.
One reason for unwanted change in the process is the gradual overcoating of the processing chamber walls during the process, causing unwanted change in the thermal, optical or electrical properties inside the chamber. For example, deposition of a dielectric layer on the chamber walls during a process may gradually change the electrical conductivity and/or electro-isolation properties of the surrounding area of the process, the reflective properties of the walls or create temperature gradients that gradually affect the quality of the thin film. Another typical example of unwanted change in the process parameters is the erosion of the sputtering target as material is removed from it, causing non-uniformity of the thickness, the chemical composition and other parameters of the deposited film.
During the last several decades, the significance and complexity of surface modification processes have been accelerating. For example, the new generation semiconductor processes bring tremendous performance advantages, but also come with new challenges associated with process accuracy and adequate process control. The new generation surface modification processes struggle with problems resulting from the lack or delay in adoption of adequate, dynamic process control capability. The current state-of-the-art process control is predominantly “after-the-fact”, based on “snapshot” metrology and run-to-run control.
The most important process control challenges that must be overcome to make the next generation thin film processes more efficient and reduce waste are the in-situ monitoring and the control of:                Extremely thin films (with thickness below 40-50 angstroms); Such thin films cannot be controlled reliably by using legacy quartz crystal monitoring, nor optically due to the lack of measurable interference fringes in the UV and visible light;        Very thick layers or multilayer stacks (with thickness over 20 μm); Such thick films and coatings also cannot be controlled by quartz crystal monitoring due to the limited amount of material that can be deposited on one or limited number of quartz chips, nor optically, due to the multitude of very sharp and dense optical interference fringes;        Compound thin films and materials. These are composed of two or more elements from different groups of the periodic table. Compound thin films and materials have a very broad use such as semiconductor and material processing technology, linear and non-linear optics, X-Ray technology, electronics, lighting, solar, superconductors, thermal and/or diffusion coatings, catalysts, bond coatings, variety of cladding materials, etc. Their chemical composition is critical for the material properties. For example control of composition of InGaN thin film quantum wells for the light emitting diodes used in the solid-state lighting industry requires accuracy better than 0.1 at. %. In-situ composition control during the process of compound material deposition is a critical enabling technology for the introduction of compound materials into daily life.        Structured 2-3D-pattern films (nanostructures, MEMS, photonic band-gap structures, thin film micro-lenses);        Special property or special profile films (metallic, absorbing, non-linear, porous, anisotropic, quintic, grad-index, etc.);        Thin film processes with very fast deposition rates (>2-5 nm/sec);        Thin films deposited on non-flat substrates or samples with odd geometries;        
Insufficient process control capabilities always result in increased product cost, due to the inherent waste of energy, material, labor and intellectual effort and delays the deployment of many important technologies. The problem becomes even more significant as the new-generation of thin film products become increasingly complex. Furthermore, in most cases both the thin film products and the deposition equipment itself are “over-engineered” to meet even tighter product specifications. For example, vacuum chambers are frequently designed larger and coatings are “overdesigned” to ensure that, despite process deviations and inaccuracies, the production will still achieve acceptable uniformity, repeatability and yield. Also, the “over-engineering” affects process scalability, time-to-market and significantly contributes to waste and inflated product cost.
Most of today's thin-film thickness monitors and deposition rate controllers implement the quartz crystal microbalance and/or photometric method of metrology. The quartz crystal monitoring (QCM) is only sensitive to the mass of the deposited film material, and cannot provide important information about the film quality, such as film composition or structure. The instrumentation is very sensitive to process temperature, temperature gradients and requires water-cooled sensors inside the vacuum chambers. Furthermore, the measured results are very sensitive to the sensor's position inside the chamber and require thorough calibration and calculation of tooling factors, specific to each deposited material. Still, the uncertainty in the QCM readout is relatively high. QCM is not practical for many applications (such as the sputtering of compound targets or alloys, ion implantation or etching).
Interferometric monitoring of multi-layers during deposition, frequently referred to as “optical monitoring” or OMS, (including monitoring reflectance, transmittance or both, at normal or oblique incidence) has other limitations. These methods are only applicable to 1) transparent films, 2) films having sufficient optical thickness to display measurable interference patterns in the visible spectrum, 3) multilayer stacks with total thickness not exceeding 5-10 μm, and 4) frequently requires equipment refurbishing. Some other solutions, such as in situ ellipsometry, RHEED, X-Ray scattering, electron diffraction and fluorescence methods are used in R&D laboratories for in situ film characterization, but require complicated theoretical fitting to retrieve actionable information. For instance, the in situ ellipsometry, frequently used in R&D settings, is slow in fitting the measured data to established models, requires very thorough calibration, expert data interpretation and can involve costly deposition chamber refurbishing. As a result, it has made little penetration as an in situ process control tool on the manufacturing floors.
Atomic Absorption Spectroscopy (AAS) is a promising method for accurately determining the deposition rate by correlating the atomic flux density in the vicinity of the substrate under deposition to the attained film thickness. Being independent from the film growing on the substrate or substrate characteristics, this technique is ideal for monitoring extremely thin and opaque films at a wide range of deposition rates, as well as compound thin films and complex substrate shapes. It does not shadow the deposition cloud and is much easier to calibrate than the QCM. As a result, in situ AAS can resolve most of the problems identified above.
AAS relies on the selective absorption of photons by the free atoms in the plasma surrounding the deposited substrate and follows the Beer-Lambert law. The electrons of the atoms can be promoted to more energetic states for a short period of time by absorbing a defined quantity of energy (electromagnetic radiation of a given spectral linewidth). This amount of energy is specific to a particular electron transition in a particular atomic element. The radiation flux through the atomic region is measured both without and with a sample present using a detector, and the ratio between the two values (absorption) is converted to detect the presence or analyze the amount of atoms (i.e. atomic concentration or mass). The measured absorption value is proportional to the atom flux density and can be used to derive calibration functions related to the rate of deposition or etching of the substrate or other physical parameters of the modified layers. Typically, AAS uses specific wavelengths of UV light that correspond to the specific absorption spectra of the element being monitored. The absorption lines are typically very narrow (referred as spectral linewidths) and lie mostly in the DUV and UV spectrum. The light sources are typically hollow cathode lamp sources (HCL) with a cathode that is identical to the element that is monitored. Multi-element HCL sources with 2-3 elements are also available.
FIG. 1 is a functional block diagram of the prior art ACCUFLUX® in-situ atomic absorption spectroscopy thin film process monitoring system marketed by SVT Associates in Eden Prairie, Minn. The thin film deposition chamber 101 comprises a substrate holder 102, which can have a variety of shapes and can perform different movements during deposition such as simple rotation, planetary rotation, translational movement or others. The substrate holder 102 can support one or more substrates 103 arranged in variety of configurations such as circular, rectangular or others. The material vapor cloud (i.e. plasma) that forms a deposition region 104 is directed towards the substrate by a deposition source 105 or multiple sources as per the materials to be deposited on the substrate. A person skilled in the art recognizes that multiple deposition configurations (vertical, horizontal, up-down, down-up, etc.) and variety of processes (physical, chemical, reactive, ablative, implantive, radiative, etc.) exist, which can all be represented by the configuration illustrated in FIG. 1.
The in-situ atomic absorption spectroscopy system is installed outside the processing chamber and generically consists of element-specific hollow cathode light sources 109 that contain the atomic elements identical to the atomic elements that are measured, beam shaping optics (i.e. focusing lenses) 110 focusing the beam inside the deposition region 104 in the chamber 101 through a tube 111, which is installed on the first optical viewport 112 of the processing chamber 101. The beam is split into two beams by an optical beam splitter 113. One of the beams is directed through an aperture 114 for power reference (referencing beam) and the second beam is directed to the processing chamber 101 for plasma measurement (probing beam). The reference and the probing beams are filtered by a mechanical chopper with a filter 115 and directed to the optical detector 116 (i.e. photomultiplier tube) by means of a focusing or other beam shaping optics 110. The probing beam enters the processing chamber through the first optical viewport 112 installed on the chamber walls, traces the deposition region 104, leaves the processing chamber through a second optical viewport 117 on the opposite wall of the processing chamber and is reflected back into the second optical viewport 117 by the beam retro-reflector 118. It again enters the processing chamber through the second optical viewport 117 and probes the deposition region 104 for a second time before it leaves through the first optical viewport 112. Once back from the measurement, the probing beam is reflected by the beam splitter 113 to the mechanical chopper 115 and to the photodetector 116 though the focusing optics 110.
The computer 106 processes and displays the measurement information to the operator. Optionally, 106 can communicate to the processing chamber through one or more controllers 107, which may control a variety of sub-systems such as mechanical shutters 108, deposition sources 105, vacuum pumps, heaters, substrate rotations, etc. as well as a variety of process parameters such as gas flow, bias voltage, temperature, etc. In some cases 106 and 107 might be integrated into one computer system.
This configuration of AAS monitoring system uses UV-transparent optical ports for the incident and the reflected beam, mechanical modulation (chopper) and a photo-multiplier or other optical detector. The probe beam travels not only over the substrate area, but also travels considerable distances L1 and L2 between the substrate and the chamber walls.
U.S. Pat. No. 8,541,741 discloses a bench-top AAS system, in which no deposition chamber is used. However, AAS measurements are made with the assistance of optical fibers for transmitting and receiving signals.